Back-thinned silicon arrays are used in various photographic systems. For example back-thinned silicon CCDs are used today in various camera systems as is described for example in U.S. Pat. Nos. 4,760,031, 4,822,748 and 4,687,922 and back-thinned silicon CMOS structures are being used with particular success in various night vision systems. One such application is the use of such an array in cameras that reproduce images using bombardment by electrons of pixel sensors. This system is described for example in U.S. Pat. Nos. 6,285,018, 6,657,178 and 6,507,147.
Cameras to create images based on electron bombardment represent, among other things, a new generation of high performance night vision video sensors. Their light-weight and compact size match the requirements for head mounted night vision systems. Such cameras have an effective spectral response that results from the sum of both the responses of their photocathode and the underlying pixel sensors. One such sensor designed for night vision applications would typically employ a GaAs photocathode and a silicon-based active pixel sensor to sense and read out the image data. As a consequence of this choice, such sensors have a spectral response that ranges from just short of 400 nm to ˜1100 nm where silicon's response falls off. The long wavelength response of these type sensors may be extended through the use of a wavelength up-conversion material.
Certain established principles have been applied to the design of light sensing detectors that make use of up-conversion materials. In order to obtain the highest possible signal to noise ratio (SNR), the wavelength-shifting material is typically the first element in the optical chain that has significant absorption at the wavelength that is to be detected. In the case of photocathode-based devices, the material is best placed as an under-layer between the input window and the photocathode. This position insures that all the photons available within the up-conversion materials usable wavelength range are efficiently used. Furthermore, the tight coupling to the photocathode insures that photoelectrons based on the up-conversion-material's output are efficiently generated allowing subsequent photoelectron gain. The combination of efficient use and gain, results in the best possible detector SNR. The same principle applies to bare silicon focal plane arrays coated with up conversion materials. The silicon surface upon which the light is incident is coated with the up-conversion material. On a front-surface silicon focal plane device the front of the sensor is coated with up-conversion material resulting in an extended wavelength response detector with reasonable sensitivity. However, due to the overlying gate structures in CCDs and the overlying metal structures in CMOS focal plane arrays, additional performance can be obtained by backside thinning the sensor and taking light in the backside of the device. Backside thinning is a costly process. However, properly designed back-thinned silicon focal plane arrays can detect light effectively over 100% of their back surfaces, often a factor of ˜2× greater sensitivity than their front side counterparts. Consequently, in cases where performance is paramount, backside thinned silicon focal plane arrays can be used in conjunction with up-conversion materials. In this case, the logical placement of the up-conversion material follows the incoming light to the backside of the device thereby avoiding the scattering and transmission losses associated with first traversing the focal plane array. This said, a different set of selection criteria apply to the sensors envisioned and described in this invention. Specifically, operational benefit can be obtained through incorporation of a low level of added long wavelength sensitivity to high performance night vision sensors in military and law-enforcement applications. The main goal of this additional response is not to add to the overall low light level performance of the device but rather to allow the detection and imaging of relative high levels of 1100–2000 nm light, generally considered near infrared light, within night-time imagery when such illumination is present. In these sensors, considerable expense has been expended to obtain the best possible low light level performance. Coating the back surface of the back-thinned silicon focal plane array with an up-conversion material results in scattering, reflection and transmission losses to the light required to form the night vision image. In the case of electron bombarded back-thinned silicon focal plane arrays, a layer of up-conversion material on the back surface would unacceptably block and scatter incoming electrons. Consequently, although there are additional losses at long wavelengths, this invention places the up-conversion material to the front surface of the back-thinned silicon focal plane array.
An alternate class of night vision focal-plane sensors incorporates gain within a specially modified CCD. E2V's CCD65 and Texas Instruments TC285SPD exemplify this class of sensors. These CCDs are well suited for night vision applications. Improved low light performance can be achieved on CCDs through backside thinning and antireflection coating. E2V's selection guide shows that CCDs can be obtained with a variety of options including backside thinning and phosphor coating in order to extend the wavelength response of the CCD into the UV or X-ray wavelength range. In each case though, the phosphor coating resides on the surface through which the incoming light enters. This is an obvious placement for a phosphor conversion layer; it maximizes conversion efficiency for the wavelengths served by the phosphor. In this invention a phosphor conversion layer is also used. However, the phosphor used with a backside thinned silicon, is positioned at the surface of the silicon opposite the side exposed to incoming light on a backside thinned CCD. Although less of the incoming light is transmitted to the phosphor layer when the layer is positioned under the thinned silicon, the first pass light directly detected by the CCD is not subject to the reflective, absorptive and scattering affects of the phosphor layer. This positioning is further facilitated by the optical transmission of silicon based CCDs within the 1100–2000 nm range. Whereas UV light is completely absorbed within the CCD, near IR light in the 1100–2000 nm range is only lightly absorbed. Consequently, the proposed placement of the layer results in undiminished performance in the back-thinned CCDs primary detection while retaining most of the potential performance of the up-converting phosphor. It is a further goal of this invention that the up-conversion material and associated layers be compatible with the high-temperature processing required to fabricate ultra-high vacuum (UHV) electron bombarded sensors.